[FIX] sale(_timesheet): faulty computation

The 'delivered quantity' field sometime needs to trigger a search
on related objects (e.g. analytic lines) to know how much has been
delivered.

For performance reasons, this search is done using a read_group where
the results are grouped by SO line.

Unfortunately, this computation is sometimes launched during
recomputes/onchanges where the line in question has a NewId instead
of a regular id.

This means that the mapping will return something like
{240: 50.0}
indicating that (e.g.) there are 50 timesheet hours for the SO line
with id 240; unfortunately by doing
mapping.get(line.id, 0.0)
the NewId (240) is used - therefore returning a delivered quantity
of 0.0 for a line which has actual timesheets.

This commit ensures that if the line has a NewId and an original
underlying record, the id of the latter is used as the key to fetch
the value from the dict that contains the read_group result.

Note that the problem was quite tricky to reproduce as it depended
on the order of operations launched by triggers - the order of
operations was important since triggering the compute with the 'real'
record put things in order before any other piece of code could have
spotted the issue.

closes odoo/odoo#39778

Signed-off-by: Damien Bouvy (dbo) <dbo@odoo.com>